1. Field
The present invention relates to a reduction gearing with a high reduction ratio according to the preamble of independent claim 1. Such a gearing comprises a ring gear, at least one planet gear rolling around in the ring gear having a planet gear axis, and an output being rotatably mounted coaxially to the ring gear.
2. Description of Related Art
A reduction gearing of the type mentioned is the well-known planetary gearing. A planetary gearing generally comprises a plurality of planet gears that mesh with both the outer ring gear as well as with an inner sun gear of the planetary gearing. The planet gears are generally rotatably mounted in a planet carrier. The planet carrier itself is aligned coaxially to the ring gear and the sun gear and likewise rotatably mounted. A planetary gearing basically offers the possibility to hold either the ring gear or the planet carrier or even the sun gear, where the other two components form the drive and the output of the gearing. It is common to use the ring gear as the fixed system and to connect the sun gear with the drive shaft. The planet carrier in this case forms the output of the planetary gearing.
High reduction ratios can already be realized with a conventional planetary gearing, in particular when the planetary gearing comprises several stages. If the planetary gearing is embodied in a multi-stage manner, however, more installation space is required in the axial direction. Some applications, however, require highly reduced gearings which may in the radial direction require somewhat more installation space, but must in the axial direction be extremely short. There are already several gearing types known that are used in such cases which are generally designed having a single stage. These gearing types include, for example, the ellipto-centric gearing, known as “Harmonic drive”. Such a gearing is known from DE 10 2008 061672 A1. Such a Harmonic drive is composed of three components, namely an elliptical drive disk, the so-called wave generator, an elastic transmission sleeve with external gear teeth, the so-called flex spline, and a rigid cylindrical outer ring with internal gear teeth, the so-called circular spline. All three components are arranged concentrically to one another. The elastic transmission sleeve always adapts itself to the external contour of the elliptical drive disk. Rotation of the drive disk therefore effects a continuous deformation of the elastic transmission sleeve. The elliptical shape causes the external gear teeth of the elastic transmission sleeve to be in engagement with the internal gear teeth of the rigid cylindrical outer ring only at two opposite sections. By rotating the elliptical drive disk, a circumferential displacement of the engagement occurs between the internal gear teeth of the elastic transmission sleeve and the outer gear teeth of the rigid cylindrical outer ring. The reduction is obtained in that the external gear teeth of the elastic transmission sleeve are fewer teeth than the internal gear teeth of the outer ring. Strain wave gearings are known to be robust and reliable. However, the elastic transmission sleeve requires the use of special wear-resistant materials, which makes the production of such gearings relatively expensive. In addition, the permanent deformation of the elastic transmission sleeve leads to lossy transmission.
Another gearing type with a high reduction ratio is the cycloidal drive, also known as an “eccentric drive”. Such a gearing is known, for example, from DE 10 2004 048197 A1. In this type of gearing, an eccentric drives a cam disc having a certain number of cam sections on the outer perimeter. The cam disk rolls around a stationary pin ring. The number of pins exceeds the number of cam sections by exactly one pin. With every rotation of the drive gear, the output moves on by one cam section. Cycloidal drives as well are extremely robust and durable. Due to the wobbling motion of the cam disk, however, vibrations can sometimes occur that can have a particularly negative impact on the degree of efficiency of the gearing. Cycloidal drives are also relatively expensive to manufacture.
A Wolfrom gearing is to be mentioned as a further gearing type with a high reduction ratio. This gearing type is a special form of a conventional planetary gearing. A Wolfrom gearing is known, for example, from DE 10 2010 0128 79 A1.
All of the aforementioned types of gearings have the disadvantage that no optimum degree of efficiency can be achieved, in particular with very high reduction ratios. In addition, the aforementioned reduction gearings are also relatively expensive to manufacture.